Method of manufacturing a heat exchanger

ABSTRACT

A method for manufacturing a heat exchanger comprises the steps of forming holes in a metal tube sheet in a desired pattern, placing a sheet of brazing foil over the tube sheet, piercing the foil with a plurality of punches aligned with the tube sheet holes, thereby staking the brazing foil segments through the tube sheet holes, pressing the tube ends through the tube sheet holes and foil segments, and heating the end of the assembly until the brazing foil melts.

United States Patent [191 McElwain et a1.

[54] METHOD OF MANUFACTURING HEAT EXCHANGER [75] Inventors: Merle G.McElwain, York, Pa.; Kenneth J. Miller, Peoria, 111.

[73] Assignee: Caterpillar Tractor Co., Peoria, Ill.

[22] Filed: June 28, 1971 [21] Appl. No.: 157,169

[52] US. Cl ..29/l57.3 R, 29/157.4, 29/47l.1, 29/501, 29/DlG. 4 [51]Int. Cl. ..B2ld 53/02 [58] Field of Search ..29/l57.3 R, 157.4, 202 D,471.1,

29/47l.3, 501, DIG. 4

[5 6] References Cited UNITED STATES PATENTS 1,420,182 6/1922 Cleveland..29/l57.4 X

[ 1 Jan. 16, 1973 Rankin ..29/D1G. 4

2,520,821 8/1950 Woods et al.. ..29/157.3 R X 3,259,003 7/1966 Griffin..29/l57.3 R UX 3,349,465 10/1967 LaPan et al. ..29/157.3 R UX PrimaryExaminerCharles W. Lanham Assistant Examiner-Donald C. Reiley, Ill 7Attorney-Charles M. Fryer et al.

[5 7] ABSTRACT A method for manufacturing a heat exchanger comprises thesteps of forming holes in a metal tube sheet in a desired pattern,placing a sheet of brazing foil over the tube sheet, piercing the foilwith a plurality of punches aligned with the tube sheet holes, therebystaking the brazing foil segments through the tube sheet holes, pressingthe tube. ends through the tube sheet holes and foil segments, andheating the end of the assembly until the brazing foil melts.

4 Claims, 6 Drawing Figures BACKGROUND OF THE INVENTION Many differenttypes of heat exchangers are in operation wherein fluid-carrying tubesare arranged in spaced parallel relationship within the exchangers. Asecond fluid is directed through the exchanger in a manner designed toprovide maximum contact of the fluid with the tubes, resulting in atransfer of heat from the hotter to the cooler fluid. Such heatexchangers include radiators, air-to-oil transmission coolers, platetypecoolers and many others. One specific type of heat exchanger, referredto as a liquid-to-liquid shell-andtube exchanger, comprises a pluralityof tubes held in a matrix by tube sheets through which the tube endsproject. The tube ends are sealed into the tube sheets by a number ofmethods including rolling, brazing or welding. The tube sheets in turnare fixed within a heat exchanger shell in fluid tight relationship bybrazing or welding.

Manifolds at the ends of the shell communicate fluid to the tubes.Generally, the cooling fluid is directed longitudinally inside thetubes, while the fluid to be cooled is directed across the outersurfaces of the tubes. Baffles are spaced at desired locations withinthe heat exchanger to insure that the main flow stream of the fluid tobe cooled is nearly perpendicular to the tube lengths, and that thefluid to be cooled repeatedly contacts the tubes.

Materials frequently used in manufacturing heat exchangers include castiron, steel, brass, copper, stainless steel and aluminum, and selectionof the material for a given application depends on the desired inherentheat transfer characteristics, corrosive environment, temperature,galvanic action and economics.

One problem frequently encountered in the construction of heatexchangers of the shell-and-tube type is in maintaining a leak-proofconnection between the tubes and the tube sheets. In a typicalexchanger, as many as five hundred individual tubes may be used,requiring formation of one thousand leak-proof connections consideringthat both ends must be sealed.

One method of fastening the individual tube ends in the tube sheetscomprises expanding the tube ends into the tube sheet holes by rolling.This mechanical bond is often unsatisfactory because the tube may workloose from, the tube sheet after prolonged operation with repeatedcontraction and expansion of the elements.

Another method of securing the tube ends in the tube is by welding.Welding the tube end connections may be satisfactory if all of thematerials used are similar. However, the high temperature used inwelding may create residual stresses in the normally thin-walled tubes,causing failure during operation.

Conventional brazing processes are largely performed by hand andconsidered to be an individual art. Thus, there is a large variance incooler quality and consistency on a day-to-day, individual-to-individualbasis.

Irregularity in heating and applying the brazing materials is likely tocause local elongation of individual tubes. When the braze solidifies,this freezes the tube in an extended position and creates a state ofresidual stress, which can result in reduced fatigue life.

2 BRIEF SUMMARY OF THE INVENTION The present invention provides a methodof constructing heat exchangers which overcomes the foregoing problems,and which assures precisely constructed,

uniform and leak-proof braze joints between the tubes 7 and the tubesheets. This method also utilizes a minimum amount of brazing materialand is readily adaptable to automated manufacture of the heatexchangers.

In the process of the present invention, the brazing material isprovided in the form of a thin sheet of brazing foil which issuperimposed on a tube sheet having holes formed therein to receive theends of cooling 5 tubes.

A plurality of punches, preferably having pointed and serrated tips, arethen pushed through the foil and into the holes in the tube sheet. Thefoil overlying the tube sheet holes is thus pushed into the holes andformed around the periphery thereof by the punch.

The punch is then withdrawn and the tube ends are inserted in thedressed tube sheet holes and the resulting tube bundle is inserted intoa heat exchanger shell.

The end of the exchanger assembly is then heated, as in.

a quartz lamp furnace, which causes the foil to melt and flow around thetubes, filling the spaces between the tubes and the tube sheet holeperimeters. The radiant heat from the furnace draws the melted brazingfoil through the space between the tubes and the holes in the tube sheetby capillary action, forming a bead around each tube. An additional beadof brazing material is placed around the perimeter of the tube sheet andserves to braze the tube sheet to the shell. A suitable manifold is thenattached to each end of the shell to complete the heat exchanger.

It is, therefore, an object of this invention to make a superior qualityheat exchanger by using an improved method of manufacturing andassembly.

A further object of this invention is to provide a method of assembly ofheat exchangers by brazing, wherein the amount of brazing material usedis carefully controlled.

A still further object of this invention is to provide a uniform heatingin assembling heat exchangers to prevent undesired distortion andwarping which may result in excessive stresses.

An additional object of this invention is to make possible the automatedmanufacture of heat exchangers.

BRIEF DESCRIPTION OF THE DRAWINGS shown in FIG. 2 showing a combinationpunching and staking operation.

FIG. 4 is a view of the tube sheet and brazing foil shown in FIG. 2showing the heat exchanger tubes assembled in the tube sheet after theing operation.

FIG. 5 is a sectional view of an assembled heat exchanger as it wouldappear in a q uartz lamp heater prior to brazing thereof.

punching and stak- FIG. 6 is a fragmentary sectional view of one end ofthe exchanger following the brazing operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, ashell-and-tube type heat exchanger assembly is shown generally at 10 ascomprising a shell 12 and a tube bundle assembly 14 disposed within theshell. The tube bundle comprises a pair of circular tube end sheets 16disposed at each end of the shell, a plurality of tubes 18 extendingbetween the tube sheets, a series of spaced segmented baffles 20, and aplurality of notched tie bars 22 holding the baffle plates.

In operation, the fluid to be cooled is normally introduced through aninlet 30 in the shell and flows in a tortuous path guided by baffles toan outlet 34. The cooled fluid then is returned to the engine,transmission, brake or the like for future use.

The cooling medium is introduced into tubes 18 through a manifold, notshown, and flows longitudinally from one end of the heat exchanger tothe other. It is then discharged through a manifold, not shown, attachedto the opposite end of the exchanger. Baffles 20 are maintained in theirdesired longitudinal location by means of notches provided in tie bars22, one of which is shown at 32. It is an important factor, insuccessful heat exchanger design, that the clearance between the bafflesand the tube outside diameter be minimal to insure that there is littleleakage around the tube. This will help increase the turbulence of thecooled fluid and thus improve heat transfer. It is essential that theconnection between the tube and the tube sheet, and the tube sheet andthe shell be bonded tightly to prevent any mixing or contamination ofthe two fluids used in the heat exchanger.

The preferred steps followed in assembling the heat exchanger, accordingto the present invention, include cutting copper tubes 18 to the desiredlength and inserting the tubes through segmented baffles 20 havingpre-formed holes in any desired pattern. Tie bars 22, which maintainbaffle spacing, are then assembled over baffles 20 by means of notches32 which engage the periphery of the baffles.

Tube sheets 16 are preferably formed from flat stock by blanking theoutside diameter slightly smaller than the inside diameter of the shell.The tube sheets contain a plurality of holes, which may be formed byconventional punching means. The pattern for the holes in the tube sheetwill obviously be identical to the aforementioned pattern of thesegmented baffles.

The tube sheets are cleaned and as shown in FIG. 2, a

sheet of brazing foil 38 having a thickness of approximately 0.00l to0.006 inches and an outside diameter slightly smaller than that of thetube sheet is positioned on top ofa tube sheet in a conventional press.

As shown in FIG. 3, a series of punches 40 are arranged in the samepattern as the tube holes in the tube sheet to have their pointed,straight-serrated tips 42 pushed through the brazing foil and into theholes in the tube sheet. Each serrated punch tip 42 is instrumental inforcing the brazing foil through the holes in the tube sheet and formingangular segments 43 which curl to form a star pattern on the oppositeside of the tube sheet. It has been found that an included cone angle of30 on the serrated punch end causes the foil to curl in an optimummanner, forming a tightly staked connection. The star pattern foilsegments act to center the tube ends in the tube sheet holes.

The hole size and the brazing foil thickness are selected to insure ametal-to-metal fit when the preassembled tube ends are pressed into thetube sheet and the intermediate foil segments, as shown in FIG. 4. Ithas been found that for optimum brazing characteristics the tube endsshould extend through the tube sheet a distance not to exceedapproximately one-sixteenth inch.

The brazing foil is also commercially available in the form of a tapehaving an adhesive backing. It has been found that such brazing tape canbe applied directly to the tube sheets with good results.

The assembly is placed in the shell 12, after which the tube ends andthe tube sheet are fluxed in a conventional manner. A ring of brazingmetal 44, having a circumference approximately equal to the outsidediameter of the tube sheet, is paced on the end to be brazed, in contactwith both the tube sheet and the shell. The purpose of this ring is tobraze the tube sheet 16 to the shell 12.

One method of heating the assembly in a uniform, closely controlledmanner to permit brazing of the tubes to the tube sheet and the tubesheet to the shell is by using a quartz lamp unit 45, as shown in FIG.5. Hinged members 46 and 48 are disposed around the shell withappropriate clearances to insure that the proper temperature ismaintained during the brazing operation and open outwardly to permit theheat exchanger to be placed and removed. A plurality of quartz lamps 52and 54 in the top and side heating units, respectively, provide closetemperature control for the brazing in an efiicient and uniform manner.Approximately 1,300F is a satisfactory temperature when the shell isconstructed of grey cast iron and the tubes and tube sheets are copper.A typical annealed brazing foil which is satisfactory in this purpose iscomposed of an alloy of 45 percent silver, 15 percent copper, 16 percentzinc, 18 percent cadmium and 6 percent miscellaneous constituents suchas iron and lead. Another brazingfoil composition which is satisfactoryis sold under the trade name Sil-Fos. Appropriate cooling means as at 55using circulating water or air may be employed to keep the unit at Ireasonably low temperatures for effective equipment maintenance andoperator comfort. A glass plate 56 acts as a shield to prevent spatterfrom the flux from being deposited on the upper quartz lamps 52 whichhas an adverse effect on lamp life.

Referring to FIG. 6, when the heating unit is operative, brazing foil 38melts, travels by capillary action from the inner surface of the tubesheets 16 and completely surrounds the tubes, forming fillets 58 therearound on both the upper and lower surfaces of the tube sheet to providea fluid-tight and pressure-resistant joint. The metal brazing ring 44 ismelted simultaneously to form a bead 60 around the periphery of the tubesheet and the shell, also providing a fluid-tight joint. The brazingfoil 38 on the under side of the tube sheet also provides a tinningeffect on the under surface and prevents the melting ring 44 fromrunning down the sidewalls of the shell.

It is to be understood that in reference herein to brazing and brazingfoil, such terms are used in a general sense only and are not to belimited to what is technically defined as brazing and brazing foil, butis to be understood as including any process, such as soldering andwelding, wherein two or more parts are joined by melting a fusible metalfoil.

What is claimed is:

l. A method for manufacturing a heat exchanger wherein at least one tubeextends between and through tube sheets disposed at each end of the heatexchanger comprising the steps of:

forming a series of holes in a tube sheet in a designated pattern;

positioning a sheet of brazing foil upon said tube sheet;

punching said sheet of brazing foil to stake segments of said brazingfoil through said holes;

pressing one end of a tube through each of said holes;

and,

melting said brazing foil to braze said tube sheet to said tubes influid-tight relationship. 2. The method of 'claim 1 wherein'the meltingstep comprises:

placing at least one end of the assembly in a quartz lamp heater, and

energizing the quartz lamp heater until the brazing sheet and adjacentto the shell, and brazing said tube sheet to said shell.

1. A method for manufacturing a heat exchanger wherein at least one tubeextends between and through tube sheets disposed at each end of the heatexchanger comprising the steps of: forming a series of holes in a tubesheet in a designated pattern; positioning a sheet of brazing foil uponsaid tube sheet; punching said sheet of brazing foil to stake segmentsof said brazing foil through said holes; pressing one end of a tubethrough each of said holes; and, melting said brazing foil to braze saidtube sheet to said tubes in fluid-tight relationship.
 2. The method ofclaim 1 wherein the melting step comprises: placing at least one end ofthe assembly in a quartz lamp heater, and energizing the quartz lampheater until the brazing foil melts and forms a fillet around each tubeon each side of the tube sheet.
 3. The method of claim 1 furthercomprising assembling said tubes and tube sheet within a shell.
 4. Themethod of claim 3 further comprising placing a ring of brazing metalaround the perimeter of the tube sheet and adjacent to the shell, andbrazing said tube sheet to said shell.